Hydrocarbons are gaining wider acceptance as viable alternative blowing agents in the manufacture of rigid polyurethane foams. Due to the non-polar hydrophobic characteristic of hydrocarbons, they are only partially soluble, if not completely insoluble, in many polyols used to manufacture rigid polyurethane foams. The insolubility or poor shelf life of hydrocarbon-polyol mixtures has, to date, limited the ability of storing batches of the mixtures for use at a later time. Due to the poor solubility of hydrocarbon blowing agents in polyols, the blowing agent must be added to the polyol composition under constant agitation immediately before dispensing the foaming ingredients through a mixhead. The poor solubility of hydrocarbons also tends to lead to larger, coarser, or uneven cell structures in a resultant polyurethane foam. As is well known, the thermal conductivity of a foam generally increases with a poor cell structure. Therefore, it is critical that hydrocarbon be uniformly dispersed under constant agitation throughout the polyol mixture immediately prior to foaming in order to obtain a rigid polyurethane foam having the desired thermal insulation values.
In U.S. Pat. No. 5,391,317, Smits sought to manufacture a foam having both good dimensional stability and thermal insulation using hydrocarbons as blowing agents. This reference taught the use of a particular mixture of C.sub.5-6 alicyclic alkanes, isopentane and n-pentane blowing agents in particular molar percents, in combination with a polyol mixture made up of an aromatic initiated polyether polyol, an aromatic polyester polyol, and a different amine-initiated polyether polyol. As the aromatic-initiated polyether polyol, Smits suggested using an alkylene oxide adduct of a phenolformaldehyde resin. The particular mixture of alicyclic and isomeric aliphatic alkane blowing agents is taught by Smits as producing a foam having good thermal insulation values.
The problem of obtaining a closed cell rigid polyurethane foam having both good dimensional stability and thermal insulation at low densities was also discussed in "An Insight Into The Characteristics of a Nucleation Catalyst I HCFC-Free Rigid Foam System" by Yoshimura et al. This publication reported the results of evaluations on a host of catalysts used in a standard polyurethane formulation to test the effects of each catalyst on the thermal insulation and dimensional stability of the foam. The standard formulation used contained 40 parts by weight of a sucrose-based polyether polyol, 30 parts by weight of an aromatic amine-initiated polyether polyol, and 30 parts by weight of an aliphatic amine-initiated polyether polyol, in a 1:1 weight ratio of aromatic to aliphatic amine-initiated polyols. This formulation was selected based upon the findings that sucrose and aromatic amine-based polyether polyols exhibited poor solubilities with cyclopentane, while aliphatic amine-based polyether polyols provided the best solubility for cyclopentane. As a result, 30 parts by weight of the aliphatic amine-initiated polyether polyol was used in the standard formulation.
Others have also tried to modify the polyol components in a polyol composition in an attempt to solubilize a hydrocarbon blowing agent in the polyol composition. In U.S. Pat. No. 5,547,998 (White et al), the level of aliphatic amine-initiated polyether polyols in a polyol composition is limited to solubilize cyclopentane in the polyol composition. When reacted with an organic isocyanate, the polyol composition, comprising an aromatic amine-initiated polyoxyalkylene polyether polyol and an aliphatic amine-initiated polyoxyalkylene polyether polyol in an amount of 10 weight percent or less by weight of the polyol composition produces a dimensionally stable rigid closed cell polyurethane foam having good thermal insulation properties.
In U.S. Pat. No. 5,648,019 (White et al), the level of aromatic polyester polyols in a polyol composition is preferably limited to 18 weight percent or less to improve the solubility of blowing agent in the polyol composition. The polyol composition is preferably reacted with an organic isocyanate to produce a rigid closed cell foam having good thermal insulation and dimensional stability.
Thus, it would be desirable to provide a polyester polyol composition which has a hydrocarbon blowing agent solubilized therein which can be used to produce dimensionally stable rigid polyurethane foam having good thermal insulation properties.